The invention relates to a microprogram unit for a data processor.
Microprogram units for data processors operate relative to a microprogram store to access routines of basic micro-instructions in order to implement various instructions within the processor. The extracted micron-instructions control the states of the various units of the processor in order to execute each desired instruction. Often, the microprogram store is writable and this allows the total microprogram to be larger than the immediate microprogram store capacity so long as an appropriate overlay facility is provided for inserting microprogram material into the immediate store as required. A suitable overlay technique is disclosed in copending U.S. Pat. Application Ser. No. 488,907, now U.S. Pat. No. 3,924,245.
The fact that the microprogram store is writable also means that the microprogram material itself is easily altered. This is particularly useful both at an early design stage of a processor and later as most computer systems are continuously developed throughout their life. However, the microprogram is most conveniently arranged sequentially relative to store addresses so that the normal accessing procedure (i.e. except for jumps between routines or specific micro-instructions) is to increment the accessing address by unity between cycles. Then, any alteration to the microprogram material that results in addition or removal of micro-instructions, or reordering of the totality of micro-instruction routines will give rise to a need for renumbering the start addresses of the routines. Usually, this involves extensive alterations in the microprogram unit.